1. Field of the Invention
This invention relates to nonvolatile semiconductor memory devices and to methods of producing the same. In particular, it relates to a nonvolatile semiconductor memory device which is electrically rewritable and has floating gate type memory cells, and to a method of producing the same.
2. Description of the Related Art
As examples of nonvolatile semiconductor memory devices which can electrically write and erase data, there are known EEPROM (Electrically Erasable Programmable Read Only Memory) and a flash memory, as disclosed in, for example, "An In-System Reprogrammable 32 Kx8 CMOS Flash Memory" V. N. Kynett, et al., IEEE Journal of Solid-State Circuits Vol. 23, No. 5, October, 1988, p.p. 1157-1163.
As the flash memories are used mainly in portable equipment, it is important to apply a low power source voltage and speed up the rewriting operation.
In order to lower the voltage applied to the control gate electrode and to increase the rewriting speed, it is necessary to increase the capacitance between the control gate electrode and the floating gate electrode. Hence, it is effective to widen the area of the overlap region of the control gate electrode and the floating gate electrode. The structure of this type is disclosed in, for example, "A 1.28 .mu.m.sup.2 Contactless Memory Cell Technology for a 3 V-only 64 Mbit EEPROM", H. Kume et al., IEDM 92, 1992, p.p. 991-993
However, in order to increase the area of the overlap region of the control gate electrode and the floating gate electrode, it is necessary to increase the area of the element isolating film, which results in the increase of the memory cell size.